We discuss inductors that produce relations consistent with their given datasets.

Let $(x_1, y_1), \dots , (x_n, y_n)$ be a dataset in $X \times Y$. Let $\mathcal{R} $ be the set of all relations on $X \times Y$.

A consistent inductor $\set{G_n: (X \times Y)^n \to \mathcal{R} }_{n}$ is one for which, for all $n \in \N $, for all $D_n \in (X \times Y)^n$, $D$ is consistent with $G_n(D_n)$. In other words, a consistent inductor always produces a relation with which the dataset is consistent.

The interpretation follows. Fix a relation $R^\star$. And let every dataset “shown” to the algorithm $G_n$ be constructed by selecting elements from $R^{\star}$. In other words, every dataset is a sequence in $R^\star$. In this case, a dataset $D_n \in (X \times Y)^n$ is always consistent with $R^\star$ and so a consistent inductor will never “eliminate” $R^{\star}$. In other words, the inductor, in order to be consistent “must eliminate” every inconsistent relation.

We may “hope” to give the algorithm a “large and diverse” datset, so that several of the elements of $R^\star$ are included. In this case, the algorithm can “eliminate” many smaller relations in $\mathcal{R} $ which did not include records in the dataset.

The rub is that any dataset is consistent with the complete relation $X \times Y$. So we can often consider a set $\mathcal{H} \subset \mathcal{R} $ of relations. It is common to call this a hypothesis class, especially for the case in which it consists of functional relations.